The oxidation characteristics of sulfite and thiosulfate were examined by using thermodynamic calculations and simulated desulfurization solution experiments to investigate their difference. Subsequently, a new multistage oxidation method using oxygen-ammonium persulfate was presented and applied to the oxidation of a real desulfurization solution. The results show that the concentrations of thiosulfate and sulfite in the real desulfurization solution decrease from 48.76 and 61.76 g/L to 2.24 and 0.02 g/L, respectively, and the ammonium sulfate products obtained are white with uniform particles. In addition, compared with ammonium persulfate alone, the multistage oxidation method can reduce the ammonium persulfate addition by 37.56% and treatment cost by 28.13%.